LDPE (low density polyethylene) exhibits excellent blown film processability but relatively low stiffness and impact toughness. LDPE was made using peroxide initiated radical polymerization of ethylene and contains both short and long chain branches. The excellent processability of LDPE is believed to be due to the presence of long-chain branch structures or dendritic structures, although such structures have not been characterized analytically.
HDPE (high density polyethylene) has purely linear PE chains without any long and short chain branches. HDPE exhibits excellent stiffness but poor mechanical toughness and blown film processability.
LLDPE (linear low density polyethylene) contains only short chain branches introduced through the addition of a linear alpha-olefin co-monomer. LLDPE has a heterogeneous composition distribution and exhibits good toughness and moderate stiffness but relatively low blown film processability.
mLLDPE (metallocene catalyst polymerized linear low density polyethylene) has a homogeneous composition distribution containing only short chain branches. mLLDPE exhibits excellent impact toughness and moderate stiffness but very poor blown film processability.
One method of determining blown film processability of PE resins is through the measurement of extension hardening using an extensional rheometer. See Polym. Eng. Sci., 38 (1998), 1685-1693, which is incorporated herein by reference. LDPE can be extensionally hardened, whereas HDPE, LLDPE, and mLLDPE do not extensionally harden except for a few grades of modified mLLDPE that show weak strain hardening. Presently, in order to maximize the blown film line speed for better film quality and for cost reduction, it is a common practice to add 10 or more % of LDPE to LLDPE or mLLDPE to improve extensional hardening and blown film processability. See J. Appl. Polym. Sci., 88(2003), 3070-3077), which is incorporated herein by reference. However, the addition of LDPE to LLDPE or mLLDPE significantly diminishes their impact toughness and mechanical stiffness.
It would be desirable to have an additive for ethylene polymers that would enhance extensional hardness and blown-film processability. It would also be desirable to have an additive that allows blown-film production rates to be increased. It would also be desirable to have an additive that does not significantly diminish impact toughness and mechanical stiffness of ethylene polymers.